For many years glass bottles having threaded necks have been molded using a well standardized type of blow molding apparatus. The molds so utilized include a pair of cast metal neck ring mold halves which meet at a vertical parting plane and together form, about a vertical axis in the parting plane, an annulus having a lower central cylindrical well within which a guide ring is accommodated. Beneath the well, and of smaller inner diameter is, in each such conventional mold half, a cavity portion which is to be machined with spiral grooves, to continue from one mold half to the other and thus provide external threads on the bottle necks. It has been difficult to machine the grooves with such precision that those in one mold half will match with those of other mold halves; selective assembly and trial runs may be necessary.
Such mold halves are expected to yield approximately 300 to 350 gross bottles for each two sections of molds before requiring repair or replacement. Need for repair or replacement is limited to the lower cavity portion which has been so machined to provide the bottle threads. Even when the mold halves are new, vertical "splits"--that is, minute vertical grooves--are likely to be present in the bottles formed, leading to rejection of a fairly substantial percent (which may be in the order of 5% to 10%) of the bottles. The cause of such "splits" is not the subject of concensus; they have been thought to be due to glass release agents employed, usually containing sulfur and oil, but may be due at least in part to constituents of the cast iron molds themselves. After substantial use, portions of these release agents are seemingly baked into the grooved surface of the mold, which must then be cleaned by sand blast or vapor blast processes. Both molding the glass bottles and cleaning the molds appear to cause wear of the thread cavities. In the course of time, threads on the bottles will not meet the requisite tolerances. Thus, mold repair or replacement is an expensive and troublesome problem not heretofore met satisfactorily.